Climatic influences on development and survival of free-living stages of equine strongyles: implications for worm control strategies and managing anthelmintic resistance.
Authors: Nielsen Martin K, Kaplan Ray M, Thamsborg Stig M, Monrad Jesper, Olsen Susanne N
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary Understanding when parasite refugia are smallest on pasture is critical for preserving anthelmintic efficacy, as treatment during these periods intensifies selection pressure for resistance in strongyle populations. Nielsen and colleagues reviewed laboratory and field evidence across three climate zones to map seasonal fluctuations in free-living strongyle development and survival, identifying that refugia dwindle during winter months in cool temperate regions (such as the UK and Northern Europe) but during summer in warm temperate and tropical climates. The research reveals a stark mismatch between seasonal climate stress and actual parasite elimination: whilst adverse seasonal conditions significantly impair larval development and survival, climatic forces alone cannot effectively clear pastures between grazing seasons, meaning refugia always persist. For practitioners, this means strategic treatment timing—avoiding unnecessary dosing during low-refugia periods—becomes essential for resistance management, but blanket reliance on seasonal "natural cleaning" is misguided. The implications are substantial: farriers and vets working with horses in different geographic locations must adopt region-specific control strategies rather than universal protocols, and grazing management or targeted selective treatment should complement seasonal considerations rather than replace year-round refugia preservation.
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Practical Takeaways
- •Time anthelmintic treatments strategically based on climate zone: avoid treatment in winter (Northern climates) or summer (warm/tropical climates) when refugia are smallest, as this increases resistance risk
- •Do not rely on seasonal weather changes to naturally clear pastures of strongyles between grazing seasons—implement integrated parasite management with grazing rotation and pasture management alongside targeted treatments
- •Maintain a pool of untreated horses and use selective treatment protocols rather than blanket deworming to preserve drug efficacy and reduce anthelmintic resistance development
Key Findings
- •In Northern temperate climates, pasture refugia of strongyle free-living stages are smallest during winter, while in warm temperate and subtropical/tropical climates refugia are lowest during summer
- •Climatic influences significantly affect development and survival of free-living strongyle stages but cannot effectively 'clean' pastures from one grazing season to the next
- •Treatment should be avoided when pasture refugia are small to minimize selection pressure for anthelmintic resistance in worm populations
- •Refugia levels are the most important factor affecting the rate of resistance development to anthelmintic drugs in equine strongyle populations